docs: def get_multi_generation(self, tables, db='default'):
          """Takes a list of table names and returns an aggregate
          value for the generation"""
          generations = []
          for table in tables:
              generations.append(self.get_single_generation(table, db))
          key = self.keygen.gen_multi_key(generations, db)
          val = self.cache_backend.get(key, None, db)
          #if local.get('in_test', None): print force_bytes(val).ljust(32), key
          if val is None:
              val = self.keygen.random_generator()
              self.cache_backend.set(key, val, settings.MIDDLEWARE_SECONDS, db)
          return val